。
She loves flowers.
Literal
She [topic-は] flowers [subject-が] greatly-like is.
好き is grammatically a na-adjective, so the thing one likes is marked with が, not を. 大好き is the intensified form — '(really) love' — built by prefixing 大 ('big, much') and one of the most common ways Japanese stacks emphasis on basic affection or distaste verbs (大好き, 大嫌い). The sentence ends in である, the formal/written variant of だ, which gives the line a slightly bookish or essay-like register; in casual speech it'd be 大好きだ or 大好き(だよ).